KIEV, Ukraine — An American-backed deal to settle the crisis in eastern Ukraine fell flat on Friday as pro-Russian militants vowed to stay in occupied government buildings, dashing hopes of a swift end to an insurgency that the authorities in Kiev portray as a Kremlin-orchestrated effort to put Ukraine’s industrial heartland under Russian control.

But the agreement, reached in Geneva on Thursday by diplomats from the European Union, Russia, Ukraine and the United States, appeared to arrest, at least temporarily, the momentum of separatist unrest in Ukraine’s Russian-speaking east. Armed pro-Russian militants, who have seized buildings in at least 10 towns and cities since Feb. 6, paused their efforts to purge all central government authority from the populous Donetsk region.

It was clear all along that for the pact to have a chance of success, the Kremlin would have to pressure the militants to leave the buildings they had seized. So far, it has shown no inclination to do so, blaming the Ukrainian government for the turmoil and denying that Russia has any ties to the rebels.

Western officials said the United States planned to reassure Eastern European members of NATO by conducting company-size — about 150 soldiers — ground force exercises in Estonia and Poland. The exercises would last a couple of weeks and would most likely be followed by other troop rotations in the region.

With militants vowing to ignore the agreement but halting what had been a daily expansion of territory under their control, officials in Kiev voiced some hope that a settlement was still possible. They were skeptical, however, about Russia’s willingness to push the separatists to disarm and vacate occupied buildings.

“If Russia is responsible before not just Ukraine but the world community, it should prove it,” said Andrii Deshchytsia, the acting Ukrainian foreign minister, who took part in the Geneva talks.

In a sign of the chasm separating Russian and Ukrainian views, Russia’s Foreign Ministry issued a statement on Friday that made no mention of the pro-Russian militants driving the unrest. It said the call for militants to disarm “meant in the first place” the disarming of Ukrainian nationalist groups hostile to Russia, like Right Sector “and other pro-fascist groups which took part in the February coup in Kiev.”

The state-run Russian television channel, reporting from an occupied building in Horlivka in the Donetsk region, featured a masked gunman who pledged to “fight to the end for his convictions.” He displayed an armband emblazoned with a swastika-like symbol, which he said had been seized from supporters of the Ukrainian government.

Doubts about the Kremlin’s readiness to push pro-Russian militants to surrender their guns have been strengthened by its insistence that it has no hand in or control over the separatist unrest, which Washington and Kiev believe is the result of a covert Russian operation involving, in some places, the direct action of special forces.

“I don’t know Russia’s intentions,” Mr. Deshchytsia said, noting that during the negotiations, Russia’s foreign minister, Sergey V. Lavrov, had repeatedly asserted “that Russia was not involved.” He said Mr. Lavrov had been “cooperative and aggressive at the same time.”

Russia’s denials have stirred concerns that it went along with the agreement not to curb the turmoil in eastern Ukraine, but to blunt American and European calls for tougher sanctions that could severely damage Russia’s already sickly economy. Western sanctions have so far been limited to a travel ban and asset freeze on a few dozen individuals and a Russian bank.

Secretary of State John Kerry called Mr. Lavrov on Friday and urged Russia to ensure “full and immediate compliance” with the agreement, a senior State Department official said. Mr. Kerry, the official added, “made clear that the next few days would be a pivotal period for all sides to implement the statement’s provisions, particularly that all illegal armed groups must be disarmed and all illegally seized buildings must be returned to legitimate owners.”

Mr. Kerry also spoke with Ukraine’s acting prime minister and praised him for moving to carry out the deal, including by increasing transparency and guaranteeing amnesty for militants who disarm and leave occupied buildings. In Washington on Friday, Susan E. Rice, President Obama’s national security adviser, denounced anti-Semitic fliers distributed in Donetsk, which instructed Jewish residents to register, as “utterly sickening” and said Mr. Obama had “expressed his disgust quite bluntly.”

American officials gave no firm timeline for when they expect militants to pull back, but said it should be days, not weeks. Vice President Joseph R. Biden Jr. is scheduled to travel to Ukraine on Tuesday, which could be a moment to assess whether the agreement has yielded results.

Russia responded with fury on Friday to remarks the day before by Mr. Obama, who said that the deal offered a “glimmer of hope” but that the United States would take more punitive action if Russia did not abide by it. The Foreign Ministry criticized Washington for making “ultimatums” and for moving “to threaten us with new sanctions, which is absolutely unacceptable.”

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The diplomatic accord, while limited in scope and skirting the contentious issue of the Crimean Peninsula, was the first time Russia and Ukraine had found common ground since protests toppled a pro-Moscow government in Kiev in February, leading the Kremlin to seize Crimea and mass about 40,000 troops on Ukraine’s eastern border.

Speaking to reporters on Thursday at the White House, Mr. Obama sounded a skeptical note. “My hope is that we actually do see follow-through over the next several days,” he said. “But I don’t think, given past performance, that we can count on that, and we have to be prepared to potentially respond to what continue to be efforts of interference by the Russians in eastern and southern Ukraine.”

Militants holed up in the headquarters of the Donetsk regional administration — which they seized on Feb. 6 at the start of a well-coordinated campaign of assaults on government buildings across the region — showed no sign on Friday of abiding by the agreement. Gunmen in Slovyansk, a city in the north of the region, also seemed unwilling to leave the local police station and other seized buildings.

Denis Pushilin — the leader of a separatist group that seized the Donetsk government headquarters; proclaimed an independent nation, the Donetsk People’s Republic; and demanded a referendum on the future status of the region — said he was not bound by any commitment made by Russia. He added that he would ignore the Geneva agreement until the interim Ukrainian government, established after the Feb. 21 flight of President Viktor F. Yanukovych, disbanded.

“They did not sign anything for us. They signed for the Russian Federation,” Mr. Pushilin told reporters. “They do not ask our advice, but I believe all the actions of the Russian Federation were intended to solve the situation peacefully.”

Mr. Pushilin said he would carry on preparing for a referendum, but was coy when asked what he would recommend if people in the region voted for independence. Would he seek for the region to become part of Russia, as occurred in Crimea, or to remain part of Ukraine under a new constitution that granted greater autonomy to the country’s regions?

Mr. Deshchytsia, the acting Ukrainian foreign minister, dismissed as irrelevant Mr. Pushilin’s demand that the government in Kiev step down. He said this was not part of the agreement and added that the government would press on with a stalled “antiterrorist operation” to dislodge the militants if they refused to leave.

The government’s efforts to confront the pro-Russian militants with force came to an ignominious halt on Wednesday after members of an elite Ukrainian military unit joined militants in Slovyansk, allowing Russian flags to be hoisted on their armored vehicles.

At a news conference in Kiev, Mr. Deshchytsia declined to set a deadline to resume the military operation if the rebels refuse to disarm. He instead called on Russia to honor what he said was a commitment made in Geneva to persuade the militants to give up their weapons and leave seized buildings.

Seeking to calm separatist passions, Ukrainian leaders on Friday repeated assurances that they would, as Mr. Deshchytsia promised in Geneva, pursue changes to the Constitution that would grant more power to local councils to run their own affairs and enshrine Russian as an official language — bedrock demands in the largely Russian-speaking east. Parliament also began preparing an amnesty law, honoring a Ukrainian pledge in Geneva not to prosecute rebels if they voluntarily disarm and hand over occupied buildings.

On a visit to Donetsk on Friday, former Prime Minister Yulia V. Tymoshenko, who was jailed by Mr. Yanukovych and is now a presidential candidate in elections to be held in May, backed calls to grant Ukraine’s regions more authority. But she ruled out any “negotiations about the division of the country,” an apparent reference to Russian demands for federalization, a proposition widely seen as a ruse to divide Ukraine into a patchwork of semi-independent entities.

“We can talk only about giving the regions more autonomy,” Ms. Tymoshenko said at a news conference.

With hopes of a diplomatic settlement battered, Ukraine’s acting minister of economic development, Pavlo Sheremeta, summed up the mood in Kiev. Saying “it is much better to shoot with words, not bullets,” he described the Geneva deal as “a step in the right direction” but added, “We are not totally excited.”

A version of this article appears in print on April 19, 2014, on Page A1 of the New York edition with the headline: Insurgents Balk at Pact, Keeping Grip in Ukraine. Order Reprints|Today's Paper|Subscribe